Implement note deletion (and undo for added notes)

[Crowerade]

Undo does not seem to work when adding a note, the button is not clickable.

[yrammos]

@Crowerade this is a good observation. Added notes, more generally, cannot be deleted. We have an analogous issues and a draft PR for accidentals (#103, #127)—but not one for notes.

Implementing this is not difficult but would requires meticulous work because the note in question :

• should be erased from the SVG (unless we implement MEI refreshes in the future using Verovio);
• should be removed from the MEI score;
• should be removed from the MEI score (if it appears there as node);
• should trigger deletions of all relations of which it is a member (with propagation of that event to any meta-relations, of course).

We can add this to the backlog but should not prioritize it because it is not a common operation and expert annotators can meanwhile use a text editor to the same effect.